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An unaligned references to a field of a [packed] struct got created.
Erroneous code example:
```compile_fail,E0793
#[repr(packed)]
pub struct Foo {
field1: u64,
field2: u8,
}
unsafe {
let foo = Foo { field1: 0, field2: 0 };
// Accessing the field directly is fine.
let val = foo.field1;
// A reference to a packed field causes a error.
let val = &foo.field1; // ERROR
// An implicit `&` is added in format strings, causing the same error.
println!("{}", foo.field1); // ERROR
}
```
Creating a reference to an insufficiently aligned packed field is
[undefined behavior] and therefore disallowed. Using an `unsafe` block does not
change anything about this. Instead, the code should do a copy of the data in
the packed field or use raw pointers and unaligned accesses.
```
#[repr(packed)]
pub struct Foo {
field1: u64,
field2: u8,
}
unsafe {
let foo = Foo { field1: 0, field2: 0 };
// Instead of a reference, we can create a raw pointer...
let ptr = std::ptr::addr_of!(foo.field1);
// ... and then (crucially!) access it in an explicitly unaligned way.
let val = unsafe { ptr.read_unaligned() };
// This would *NOT* be correct:
// let val = unsafe { *ptr }; // Undefined Behavior due to unaligned load!
// For formatting, we can create a copy to avoid the direct reference.
let copy = foo.field1;
println!("{}", copy);
// Creating a copy can be written in a single line with curly braces.
// (This is equivalent to the two lines above.)
println!("{}", { foo.field1 });
}
```
### Additional information
Note that this error is specifically about *references* to packed fields.
Direct by-value access of those fields is fine, since then the compiler has
enough information to generate the correct kind of access.
See [issue #82523] for more information.
[packed]: https://doc.rust-lang.org/reference/type-layout.html#the-alignment-modifiers
[undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
[issue #82523]: https://github.com/rust-lang/rust/issues/82523
|
